GaN based HEMT (High Electron Mobility Transistor) devices are well suited as power switching devices. At the core of GaN HEMT power devices is an AlGaN/GaN heterojunction (also called barrier/channel) that confines high mobility 2DEG (two-dimensional electron gas) along its interface. HEMT devices are formed by source and drain contacts and the current is modulated by a gate voltage. To meet high breakdown voltage and low conduction loss requirements for GaN HEMT power devices, GaN epi structures must be carefully designed to provide sufficient vertical voltage blocking capability and high lateral electron mobility.
To realize GaN power devices which are cost competitive to their Si device counterparts, GaN is typically grown on 150 mm or 200 mm diameter Si substrates (GaN-on-Si) using MOCVD (Metal-Organic Chemical Vapour Deposition) reactors. Due to large differences in lattice constant and thermal expansion coefficient between Si and GaN, SiN and AlN based nucleation layers are typically grown on silicon substrates followed by multiple AlGaN transition layers with varied Al compositions to mitigate the lattice mismatch and thermal mismatch. A single GaN or AlGaN buffer layer doped with iron or carbon is deposited on the AlGaN transition layers for voltage blocking. A GaN channel layer and an AlGaN barrier layer are then grown on the single buffer layer doped with iron or carbon, so as to form the active HEMT device region where a high mobility (>1500 cm2/V·s) 2DEG can flow laterally along the AlGaN/GaN heterojunction interface.
GaN and AlGaN buffer layers become highly resistive when heavily doped with iron or carbon impurities, enabling high voltage blocking capability up to 1200V or even higher depending on doping level and buffer layer thickness. However, dopants intentionally incorporated into a buffer layer during the epi growth process to achieve high resistivity act as traps for free carriers from the 2DEG and lead to dynamic switching issues in power switching devices because of their deep level acceptor characteristics. For example, excessive high carbon concentration in a GaN epi-layer causes a dynamic switching issue known as current collapse or Rdson shift. The carbon impurities act as deep level traps which capture free carriers under high voltage stress (off-state) and lead to reduced current or higher Rdson (on-state resistance) afterwards in the on-state. This problem causes many reliability concerns with GaN HEMT-based power devices, and limits the commercialization of GaN-based power device switching technology.
Accordingly, there is a need for III-nitride semiconductor devices with both fast dynamic switching and high breakdown voltage characteristics.